The magnetic surfaces of the disks used for data storage must be extremely smooth with peak-to-valley roughness of some thin film disks being less than 100 Angstroms. The design of most disk drives allows the sliders to rest on the disk surface when the disk is not spinning. If the sliders were allowed to rest on a very smooth surface, the result would be unacceptably high stiction forces between the disk surface and the sliders. To reduce the stiction forces between sliders and smooth disks, a band on the disk surface may be deliberately roughened by, for example, laser texturing to form a contact start/stop (CSS) region.
Laser texturing of a CSS is described by Ranjan, et al., in J. Appl. Phys. 4-91, p.5746ff. The average surface roughness (Ra) was varied by altering the beam current. U.S. Pat. No. 5,528,922 describes the use of increased number of laser pulses to increase the height of a peripheral ridge in crater shaped bumps. U.S. Pat. No. 5,062,021 describes the use of laser created bumps to form a CSS region, controlling depth and height of the bumps by varying the laser power and pulse duration, and altering the shape of the bumps by varying the laser beam inclination relative to the disk surface.
A typical CSS region contains a generally spiral pattern formed from tens of thousands of bumps in a 3 mm wide annular region. Using prior art techniques laser bumps can be produced in various shapes including sombrero-like bumps, ring-shaped or dome-shaped bumps. Dome-shaped bumps are preferred for glass disks. The diameter of the bumps is typically 5 to 30 microns with spacing of 10 to 100 microns and a height on the order of a few tens of nanometers.
The laser texturing of disks takes place prior to deposition of the thin film coatings, but may follow certain preparatory steps. For example, typically AlMg substrates are electrolessly plated with NiP and polished as an initial step. Other substrates such as glass are not plated, but may require polishing. The prepared substrate is laser textured and sputtered with the appropriate layers and progressively assembled into a functional disk drive using conventional techniques.
Since both sides of a disk normally need to be textured, there is an advantage to a system which can texture the two sides simultaneously to cut the processing time more than in half, as well as to reduce disk-handling steps and associated risks of damage. Since the two sides must be extremely uniform it is required that the beams on each side be essentially identical. Beam splitting is one way to get similar beams from a single source. Acousto-optic modulators (AOMs) are often used to split and/or modulate a laser beam. The basic principle of AOMs is that a transparent medium subject to a stress undergoes a change in the index of refraction. When the stress is associated with a high frequency sound wave, such as might be produced by a piezoelectric transducer, the change in refraction is periodic. The periodic refraction pattern can act as a diffraction grating producing what is called Bragg diffraction. The undiffracted beam is called the zeroth order beam and the primary diffracted beam is called the first order beam. The amount of light diffracted in an AOM is a function of the acoustic power being applied and can, therefore, be modulated by varying the applied power. Generation of a second modulated beam using prior art techniques requires a second AOM in series with the first. The second modulated beam created this way may vary significantly from the first in intensity or size and may contain an admixture of the other modulated beam.